Valorisation of a kaolin mining waste for the production of geopolymers

This paper assesses the feasibility of using a kaolinitic sludge, which is a residue from the mining industry, is available in large volumes and is currently landfilled, as a raw material for the production of geopolymer binders. The geopolymers were synthesized at 50 °C with an overall Na2O/Al2O3 molar ratio of 1.0, and overall SiO2/Al2O3 molar ratios between 3.1 and 4.5. The content of soluble silicates, associated with the SiO2/Al2O3 molar ratio, was identified by calorimetric and spectroscopic techniques to play an important role in determining the nature of the reaction products formed. A highly dense and cross-linked Si-rich gel was identified as the main reaction product when an increased content of soluble silicates was used. Geopolymers based on kaolinitic sludge waste developed compressive strengths of up to 70 MPa after one day of curing. This elucidates that geopolymerization can be a feasible technological solution for the valorization and exploitation of kaolinitic sludge waste from the mining industry for the production of cementitious matrices.